Email File Save not the same as a file transfer?

Was trying to send a eBook to my grandson who lives in a different city, so I sent it by email. When he saved it, it would load (normally) in Calibre Book Reader, but only displayed a blank page. I had already loaded it from my Mac into my PB Calibre and it worked. When I emailed it to myself, then did a save as from the email client, I got the same result.

Does anyone know why the Save As from email would change the file? And if there is a way around it?

Series of questions: Was the book encoded with DRM? What format was it? Is there PB Calibre app? Did you convert it between formats?

Quick answers: no DRM; epub format; Calibre is called "Book Reader for epub and Kindle books" on AppWorld (catchy name don't you think?) ; no conversion between formats.

Clarification: exact same file - 1.copied in finder to PB directory - opens fine 2. Emailed from MacMail through gmail account - after save as from PB messages, loads but won't display 3. Copied file, renamed so I can see it, attached on PB Messages sent through gmail account to PB Messages, opened, save as, renamed to epub extension - works fine.
Must be a MacMail problem?